@lb150t, welcome here.
A remote diagnosis is always a little bit like cristal ball viewing, but....
-first suspect would be for me a lost gasket between cylinderhead and muffler
-second possibility could be loose bolts or nuts there which should keep your muffler happily united with mentioned cylinderhead
-third (if) there your muffler has a slip-on part, those might be separated
-fourth, CO2 sensor not tighened
-fifth, muffler neck demaged from dismantling
-sixt, simply a hole which corroded through by time and during last munipulations got opened.
Please give a feedback, any question welcome.